Abstract
Multigeneic QTL present significant problems to analysis. Resistance to soybean (Glycine max (L) Merr.) sudden death syndrome (SDS) caused by Fusarium virguliforme was partly underlain by QRfs2 that was clustered with, or pleiotropic to, the multigeneic rhg1 locus providing resistance to soybean cyst nematode (SCN; Heterodera glycines). A group of five genes were found between the two markers that delimited the Rfs2/rhg1 locus. One of the five genes was predicted to encode an unusual diphenol oxidase (laccase; EC 1.10.3.2). The aim of this study was to characterize this member of the soybean laccase gene-family and explore its involvement in SDS resistance. A genomic clone and a full length cDNA was isolated from resistant cultivar 'Forrest' that were different among susceptible cultivars 'Asgrow 3244' and 'Williams 82' at four residues R/H168, I/M271, R/H330, E/K470. Additional differences were found in six of the seven introns and the promoter region. Transcript abundance (TA) among genotypes that varied for resistance to SDS or SCN did not differ significantly. Therefore the protein activity was inferred to underlie resistance. Protein expressed in yeast pYES2/NTB had weak enzyme activity with common substrates but good activity with root phenolics. The Forrest isoform may underlie both QRfs2 and rhg1.
Highlights
Genetic studies have mapped a number of quantitative trait loci (QTL) conferring resistance to sudden death syndrome (SDS; QRfs, QRsfs1, QRfs2, QRfs3) and soybean cyst nematode (SCN; rhg1) on soybean linkage group G (LG G; chromosome 18; Iqbal et al, 2001; Triwitayakorn et al, 2005)
This paper reports the characterization of the laccase enzyme; identification of laccase alleles; analysis of transcript abundances in roots of several soybean varieties varying in partial resistance to SDS and SCN; and associations with the alleles at the receptor like kinase within the rhg1 region The possibility that laccase is a candidate for one of the genes underlying both the rhg1 and the QRfs2 locus is discussed
Map based cloning is an important tool in soybean gene identification (Searle et al, 2003; Ashfield et al, 2003; Gao et al, 2005)
Summary
Genetic studies have mapped a number of quantitative trait loci (QTL) conferring resistance to sudden death syndrome (SDS; QRfs, QRsfs, QRfs, QRfs3) and soybean cyst nematode (SCN; rhg1) on soybean linkage group G (LG G; chromosome 18; Iqbal et al, 2001; Triwitayakorn et al, 2005). The Rfs2/rhg region on LG G encompassed between SIUCSac (Ruben et al, 2006) and AFLP marker ATG4 (Meksem et al, 2001) was shown to contain five genes; a candidate receptor like kinase gene potentially involved in extracellular signal reception and intracellular signal transduction; and four genes encoding enzymes that might be involved in metabolism. Extensive soybean genome sequence from the susceptible cultivar ‘Asgrow 3244’ has been released to GenBank encompassing Rfs2/rhg (Hague et al, 2001). Extensive sequencing of the receptor like kinase in 32 cultivars and 112 plant introductions (PIs) showed there were 9 alleles encoding 5 different proteins (Ruben et al, 2006). Since linkage disequilibrium in soybean is often large (97–536 kbp; Hyten et al, 2007) significant numbers of alleles among the linked genes are expected
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